1. Field of the Invention
The present invention relates to a stator core of an electric rotating machine which has a plurality of ring-shaped steel sheets obtained from a single steel plate wound in the cylindrical shape. The present invention also relates to a method of manufacturing the core wherein each steel sheet is lengthened in the circumferential direction when the steel plate is wound into the steel sheets.
2. Description of Related Art
As an electric rotating machine generating a rotational force or electric power, a helical stator core has been generally used. To form this stator core, magnetic pole teeth are press-formed on one side of a long steel plate, and the steel plate is helically wound or bent to obtain a lamination of a plurality of ring-shaped steel sheets formed in a cylindrical shape. Each ring-shaped steel sheet has the magnetic teeth and slots alternately arranged along the circumferential direction of the sheet on the inner side of the sheet. Further, the long steel plate is wound while the other side portion of the plate is thinned in a taper shape in section by a roller or the like so as to lengthen the outer side portion of each steel sheet. Therefore, the plate can be easily wound. The thinning and winding of the plate is, for example, disclosed in Japanese Patent Specification No. 3539626. As a result, a laminated core having the ring-shaped steel sheets is obtained. The outer end portion of each ring-shaped steel sheet is thinned.
Further, after the laminated core is obtained, positions of the sheets of the laminated core are minutely adjusted in each of the radial and circumferential directions of the laminated core so as to align the sheets within a permissible range in the radial and circumferential directions. Then, the outer end portions of the sheets are partially welded to one another to fix the positional relationship of the sheets aligned in the radial and circumferential directions. Then, in a finishing process, ironing (or trimming or shaving) is performed for the laminated core. This ironing is, for example, disclosed in Published Japanese Patent First Publication No. 2006-246586. The ironing is performed to adjust the circularity of the laminated core in outer diameter, the concentricity between inner and outer portions of the laminated core, and the perpendicularity of each sheet to the inner and outer circumferential surfaces of the laminated core. Therefore, a stator core is formed from the laminated core by performing the ironing for the laminated core.
At the final step of the finishing process, the ironing is performed for the outer end portions of the sheets to set back ends of the outer end portions excessively lengthened toward the radial direction of the cylindrical core for the purpose of adjusting the outer diameter of the core. In this ironing, an ironing member of a forming apparatus is moved up and down along the axial direction of the laminated core to push the outer end portions of the sheets toward the inner side of the axial direction. Therefore, plastic deformation is caused in the outer end portions of the sheets in response to the ironing load. Ends of the outer end portions are set back in response to the plastic deformation.
However, because the outer end portions of the sheets are thinned, the outer end portions of the sheets are spaced apart from one another through openings along the axial direction. In this case, bending deformation is easily caused in the outer end portions of the sheets in the axial direction in response to the ironing load, so that the openings among the outer end portions are enlarged without setting back any bended outer end portion.
To prevent the bending deformation caused in the outer end portions, a first prior art and a second prior art are known. In these prior arts, before the ironing for the outer end portions of the sheets, the outer end portions are pushed to one another along the axial direction so as to be brought into contact with one another.
FIG. 1A is a longitudinal sectional view of an inward inclination type stator core in a first prior art, while FIG. 1B is an enlarged sectional view of outer end portions of sheets of the stator core shown in FIG. 1A. FIG. 2A is a longitudinal sectional view of an outward inclination type stator core in a second prior art, while FIG. 2B is an enlarged sectional view of outer end portions of sheets of the stator core shown in FIG. 2A.
As shown in FIG. 1A, in the first prior art, a laminated core 101 has a plurality of steel sheets 111, and each sheet 111 has an outer end portion 103 of a core back portion 102. The outer end portions 103 of the sheets 111 are thinned in the winding process. The outer end portions 103 placed on each of both outer sides (left and right sides in FIG. 1A) in the axial direction are inclined toward the outer end portions 103 placed in the center side in the axial direction. The outer end portions 103 placed in the center side of the laminated core 101 extend straight along the radial direction. Then, the ironing is performed for the outer end portions 103. Therefore, the inward inclination type stator core 101 is formed so as to have the inwardly-inclined outer end portions 103 being in contact with one another.
Further, as shown in FIG. 2A, in the second prior art, a long steel plate having a portion 118 inclined or bent toward one side of the plate is helically wound into a laminated core 101 having a plurality of steel sheets 111 while the inclined portion 118 is thinned. Therefore, each steel sheet 111 has an outer end portion 103 in a core back portion 102. The outer end portion 103 derived from the portion 118 has been thinned and inclined. Then, the ironing is performed for the outer end portions 103. Therefore, an outward inclination type stator core 101 is formed so as to have the inclined outer end portions 103 being in contact with one another.
Then, in the prior arts, a stator coil (not shown) is inserted into slots 104 of the core 101 to form a cylindrical stator, and a rotor acting as an armature is disposed in a center space of the stator so as to face the stator. Then, as shown in FIG. 1B and FIG. 2B, to fix the core 101 to a frame 109 of an electric rotating machine, the core 101 is inserted into an opening 110 such that two outer end portions 103 placed at axial ends of the core 101 come in contact with respective walls of the frame 109. The walls of the frame 109 are formed in parallel to each other and face each other through the opening 110. The core 101 is placed such that the axial direction of the core 101 is parallel to a line connecting the walls of the frame 109. Then, the core 101 is fastened to the frame 109 by through bolts or the like. Therefore, an electric rotating machine with the stator and rotor is manufactured. The stator acts as a field of the machine.
However, it is difficult to reliably fix the stator core 101 to the frame 109. As shown in FIG. 1B and FIG. 2B, the stator core 101 is forcedly inserted into the opening 110 of the frame 109 and is fastened to the frame 109. In case of the inward inclination type stator core 101, as shown in FIG. 1B, because the two outer end portions 103 placed at the axial ends of the core 101 are inclined toward the center side with respect to the radial direction, the two outer end portions 103 of the core 101 are not parallel to the walls of the frame 109. Therefore, when the core 101 is fastened to the frame 109, the two outer end portions 103 of the core 101 are not in face-to-face contact (hereinafter, called face contact) with the walls of the frame 109 but are in line-to-line contact (hereinafter, called line contact) with the walls of the frame 109. Therefore, the force for fixing the core 101 to the frame 109 becomes insufficient. Because of the insufficient fixing force, the through bolts fastening the core 101 to the frame 109 are easily loosened by external vibrations caused in the machine, and the center axis of the core 101 is shifted from the center axis of the rotor. In this case, the air gap between the inner circumferential surface of the stator and the outer circumferential surface of the rotor lacks uniformity along the circumferential direction of the core 101, and the rotational force or electric power outputted from the machine is undesirably reduced. Further, the magnetic attraction force acting between the stator and the rotor lacks uniformity along the circumferential direction, so that noise caused by the lack of uniformity in the magnetic attraction force is undesirably increased.
Further, in case of the outward inclination type stator core 101, as shown in FIG. 2B, because two outer end portions 103 placed at the axial ends of the core 101 are inclined toward one side of the axial direction, the two outer end portions 103 of the core 101 are not parallel to the walls of the frame 109. Therefore, the two outer end portions 103 of the core 101 are not in face-contact with the walls of the frame 109 but are inline-contact with the walls of the frame 109. Further, the line-contact point of one outer end portion 103 of the core 101 is placed at the top of the inclined outer end portion 103 furthest from the center axis of the core 101, while the line-contact point of the other outer end portion 103 of the core 101 is nearer to the center axis. That is, the line-contact point of one outer end portion 103 of the core 101 differs in the radial direction from the line-contact point of the other outer end portion 103 of the core 101.
In response to the difference of the line-contact points in the radial direction, not only the force for fixing the core 101 to the frame 109 becomes insufficient, but also a rotational moment M is generated in the core 101. As the fastening force of the through bolts is increased, this rotational moment M is increased. The moment M causes each inclined outer end portion 103 to extend straight along the radial direction. Further, the moment M causes magnetic pole teeth 112 integrally connected with the portions 103 in the core 101 to be raised toward one side of the axial direction and to be inclined. In this case, the stator coil inserted into the slots 104 of the core 101 is deformed or damaged, so that the insulation performance of the stator coil covered with an insulation film is undesirably lowered. Further, the air gap between the stator and the rotor is enlarged in the radial direction, so that the output of the machine is sometimes reduced.
To solve these problems caused by the line-contacts of the core 101 with the frame 109, Published Japanese Patent First Publication No. H5-23754 (1993) discloses a stator wherein portions of both end surfaces of a core with inclined outer end portions are cut to form the end surfaces extending in perpendicular to the axial direction of the core. However, it is troublesome to cut the end surfaces. For example, a cutting machine and a cutting tool are required, and it takes a long time to cut the end surfaces. Therefore, the manufacturing cost of the machine is undesirably increased.